The present invention relates in general to injection molding machines, and in particular to a new and useful injection molding machine having means for increasing the pressure of gas supplied to a mold of the machine.
Injection molding machines are known where, during injection of plastic into a mold ("the molding tool") or respectively after injection of the plastic and before the mold is opened, a gas, preferably nitrogen, is injected or blown into the mold. Since this gas is injected into the not yet hardened plastic, the gas forms a cavity in the molded piece, which leads to an often significant savings of material, in particular in the case of very voluminous pieces.
In such known devices, as described for instance in British patent document GB-A-2,139,548, exactly that quantity of gas that is intended to be forced into the mold is supplied to the piston reservoir. By displacement of the piston of the piston reservoir, this defined quality of gas is brought to the pressure at which the gas is to be introduced into the mold. As the quantities to be introduced depend entirely on what object is to be produced, and also the injection pressure of the gas varies within wide limits, for example between 40 and 200 bar, the starting position of the piston of the piston reservoir is different from case to case.